1. Field of the Invention
The invention relates to the manufacture of hot-rolled or cold-rolled steel parts having, after hot-forming or quenching in a tool, particularly favorable and homogeneous strength, elongation and corrosion-resistance properties.
For some applications, the aim is to produce steel parts that combine high tensile strength, high impact resistance and good corrosion resistance. This type of combination is particularly desirable in the automobile industry in which significant lightening of vehicles is sought. This may in particular be obtained thanks to the use of steel parts having high mechanical properties, the microstructure of which is martensitic or bainitic-martensitic: anti-intrusion parts, structural parts or parts contributing to the safety of motor vehicles, such as fender cross-members and door or center pillar reinforcements for example, require the above properties.
2. Description of the Related Art
Thus, patent FR 2 780 984 discloses an aluminum-coated steel sheet having very high-strength after heat treatment, this treatment comprising heating between Ac3 and 1200° C. followed by hot-forming within a tool. The elements of the composition of the steel are the following: C: 0.15-0.5%; Mn: 0.5-3%; Si: 0.1-0.5%; Cr: 0.01-1%, Ti:<0.2%; Al, P:<0.1%; S:<0.05%; B: 0.0005-0.08%. The alloyed compound formed by interdiffusion between the precoating and the steel during the heat treatment provides protection against decarburation and corrosion.
In one embodiment, the use of a steel comprising 0.231% C, 1.145% Mn, 0.239% Si, 0.043% Al, 0.020% P, 0.0038% S, 0.179% Cr, 0.009% Cu, 0.020% Ni, 0.032% Ti, 0.0042% N, 0.0051% Ca and 0.0028% B makes it possible to obtain, after hot-forming, a strength greater than 1500 MPa associated with a completely martensitic structure.
The downside of this very high-strength level is the elongation at break, which is relatively low after heat treatment, around 5%. However, certain applications do not need such a high-strength level, but on the other hand they require an elongation at break capability in excess of 15%. These applications also require good corrosion protection of the parts.
These structural applications relate to reinforcing parts with a thickness between 0.5 and 4 mm approximately. What is sought is a steel having a strength and an elongation at break, after heat treatment of the parts, greater than 500 MPa and 15% respectively. The combination of these mechanical properties ensures, in the event of an impact, high energy absorption. These strength and elongation requirements must be satisfied even if the cooling rates within a tool may differ between parts 0.5 mm in thickness and those about 4 mm in thickness. This situation for example would allow the adjustments of an industrial line comprising both heating and cooling not to be changed when parts of different thickness, within the above thickness range, are treated in succession on the line.
Moreover, it is known that the hot-stamping of a part or its quenching between tools may result in local deformation of relatively large magnitude in certain regions. The contact between the part and the tool may be more perfect, or less perfect, to such an extent that the cooling rate may not be the same at every point. These local variations in terms of degree of deformation or of cooling rate may have the consequences that, after heat treatment, the part has a heterogeneous structure and non-uniform properties.